U.S. patent number 4,784,466 [Application Number 06/908,463] was granted by the patent office on 1988-11-15 for optical transmission system comprising a radiation source and a multipleclad monomode optical transmission fibre with a negative-step index profile.
This patent grant is currently assigned to U.S. Philips Corp.. Invention is credited to Peter K. Bachmann, Giok D. Khoe.
United States Patent |
4,784,466 |
Khoe , et al. |
November 15, 1988 |
Optical transmission system comprising a radiation source and a
multipleclad monomode optical transmission fibre with a
negative-step index profile
Abstract
An optical transmission system is described which comprises a
radiation source (30) and a multiple-clad monomode optical
transmission fibre (10) with a negative-step index profile. In
order to improve the efficiency of coupling the radiation from the
source into the transmission fibre a single-clad-monomode
transmission fibre (20), whose end portion which faces the source
is tapered, is arranged between the source and said fibre.
Inventors: |
Khoe; Giok D. (Eindhoven,
NL), Bachmann; Peter K. (Aachen, DE) |
Assignee: |
U.S. Philips Corp. (New York,
NY)
|
Family
ID: |
19846616 |
Appl.
No.: |
06/908,463 |
Filed: |
September 17, 1986 |
Foreign Application Priority Data
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Sep 26, 1985 [NL] |
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8502625 |
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Current U.S.
Class: |
385/33; 385/127;
385/43 |
Current CPC
Class: |
G02B
6/4203 (20130101) |
Current International
Class: |
G02B
6/42 (20060101); G02B 006/22 (); G02B 006/26 () |
Field of
Search: |
;350/96.15,96.30,96.33,96.18 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
"Electronics Letters", Mar. 17, 1983, vol. 19, No. 6, pp. 205-207.
.
The Bell System Technical Journal, May-Jun. 1977, pp. 703-718-Loss
Analysis of Single-Mode Fiber Splices. .
Journal of Lightware Technology, vol. LT-2, No. 3, Jun. 1984, pp.
217-227: Progress in Monomode Optical-Fiber Interconnection
Devices. .
Applied Optics, vol. 19, No. 15, pp. 2578-2583/Aug. 1, 1980,
Efficient Coupling from Semiconductor Lasers into Single-Mode
Fibers with Tapered Hemispherical Ends..
|
Primary Examiner: Sikes; William L.
Assistant Examiner: Ullah; Akm E.
Attorney, Agent or Firm: Botjer; William L.
Claims
What is claimed is:
1. An optical transmission system comprising a radiation source; a
multiple-clad monomode optical transmission fibre with a
negative-step index profile; and a single-clad monomode optical
transmission fibre, having a tapered core characterized by a first
refractive index, arranged between the multiple-clad monomode
optical transmission fibre and the radiation source.
2. An optical transmission system as claimed in claim 1, wherein
the single clad fibre and the multiple clad fibre are characterized
by a substantially equal spot size at an interface
therebetween.
3. An optical transmission system as claimed in claim 1 or 2,
further comprising a lens system interposed between the radiation
source and the single-clad monomode optical transmission fibre.
4. An optical transmission system as claimed in claim 3, wherein
the lens system comprises a transparent material which is applied
to one end of the single-clad monomode optical transmission fibre
and which has a spherical outer surface.
5. An optical transmission system as claimed in claim 4, wherein
the lens material is characterized by a higher refractive index
than the refractive index of the core material of the single-clad
monomode optical transmission fibre.
6. An optical transmission system as claimed in claim 4, wherein
the lens material has a refractive index which is substantially
equal to the refractive index of the core material and in that the
lens material has a melting temperature which is lower than that of
the transmission fibre material.
Description
FIELD OF THE INVENTION
Background of the Invention
The invention relates to an optical transmission system comprising
a radiation source and a multiple-clad monomode optical
transmission fibre with a negative-step index profile.
Such a fibre differs from more conventional fibres in that the
cladding does not comprise a single layer but a plurality of layers
having mutually different refractive indices. The refractive index
profile of the fibre is a curve representing the variation of the
refractive index in a radial direction. A negative step in the
refractive index profile means that one of the cladding layers has
a lower refractive index than the nearest cladding layer
surrounding it.
British Patent Application No. 2,116,744, which has been laid open
to public inspection, describes a quadruple-clad monomode
transmission fibre whose cladding comprises four layers having
different refractive indices. At least one of the layers has a
refractive index which is lower than the refractive index of the
layer surrounding it. In comparison with a conventional monomode
fibre comprising a single-layer cladding, such a fibre has the
advantage that the chromatic dispersion remains low over a larger
wavelength range. As a result of this, said fibre has a higher
transmission capacity than a conventional monomode transmission
fibre. The transmission capacity is the amount of information that
can be transmitted per unit of time.
A familar problem encountered when using optical transmission
fibres is the low coupling efficiency, i.e. the fraction of the
radiation emitted by the radiation source which is not transmitted
by the transmission fibre. As is known, inter alia from the article
"Efficient coupling for semiconductor lasers into single-mode
fibres with hemispherical ends" in "Applied Optics", Vol. 19, No.
15, pages 2578-2583, August 1980, the coupling efficiency of a
single-clad monomode optic transmission fibre can be improved if
the core and suitably also the cladding are tapered at the fibre
end portion.
When this measure is applied to, for example, a quadruple-clad
monomode transmission fibre this often does not yield the
anticipated increase in coupling efficiency. Measurements on a
quadruple-clad transmission fibre having a tapered end portion
reveal that the coupling efficiency is only half the coupling
efficiency of a single-clad monomode transmission fibre having a
tapered end portion. It has been found that as the diameters of the
fibre core and cladding are reduced the fundamental mode is
decreasingly confined to the core and leaks out of the fibre. This
prohibits an efficient transmission of radiation from the light
source to the straight part of the quadrupleclad optical
transmission fibre. The above effect may occur in any fibre whose
refractive index profile exhibits a negative step.
It is the object of the present invention to provide an optical
transmission system in which the radiation is coupled from the
source into such a fibre with an optimum efficiency.
To this end the optical transmission system in accordance with the
invention is characterized in that a single-clad monomode optical
transmission fibre, whose end portion which faces the radiation
source has a tapered core, is arranged between the multiple-clad
monomode optical transmission fibre and the radiation source.
The coupling efficiency can be optimized by a suitable choice of
the parameters of the tapered end portion of the single-clad
monomode transmission fibre.
The invention is characterized further in that the spot size at the
straight end of the single-clad transmission fibre is substantially
equal to the spot size of the multiple-clad optical transmission
fibre.
As is stated in the article "Loss Analysis of Single Mode Fibre
Splices" by D. Marcuse in "The Bell System Technical Journal" 1977,
pages 703-718, the radiation field of a monomode transmission fibre
bears much resemblance to a Gaussian intensity distribution.
Therefore, it is possible to assign a mode spot to such a fibre,
which spot is characterized by a spot diameter 2W. W is the
distance between the point of maximum intensity within the spot and
the point where the intensity has decreased to 1/e.sup.2 of the
maximum value. The mode-spot diameter 2W is the principal parameter
in the case of coupling into a monomode transmission fibre. The
desired spot size of the single-clad monomode transmission fibre,
adapted to the spot size of the multiple-clad monomode transmission
fibre, can be obtained by a suitable choice of the difference in
refractive index between core and cladding materials and of the
core diameter, as is described in said article by D. Marcuse.
In order to obtain a further increase in coupling efficiency the
transmission system may be characterized further in that a lens
system is interposed between the radiation source and the tapered
end portion of the single-clad monomode transmission fibre.
In accordance with a further characteristic feature said lens
system suitably comprises a transparent material which is applied
to the end of the single-clad monomode transmission fibre and which
has a spherical outer surface.
It is to be noted that such a tapered monomode transmission fibre
with integrated lens is known per se from the article "Efficient
coupling of laser diodes to tapered monomode fibers with high-index
end" in "Electronics Letters" Vol. 19, No. 6, March 1983, pages
205-207. However, in the device described in said article the
single-clad monomode transmission fibre is not combined with a
quadruple-clad monomode transmission fibre.
BRIEF DESCRIPTION OF THE INVENTION
In a first embodiment the transmission system comprising a
single-clad tapered monomode transmission fibre with integrated
lens is characterized in that the refractive index of the lens
material is higher than that of the core material of the
single-clad monomode optical transmission fibre.
In a second embodiment this transmission system is characterized
further in that the lens material has a refractive index which is
substantially equal to that of the core material and in that the
lens material has a melting temperature which is lower than that of
the transmission fibre materials.
This second embodiment has the advantage that reflections from the
transitions between the lens material and the fibre material are
minimal, because the refractive indices of the materials are
substantially equal.
DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described in more detail,
by way of example, with reference to the accompanying drawing, in
which:
FIG. 1 shows a known quadruple-clad optical transmission fibre,
and
FIG. 2 shows an optical transmission system in accordance with a
preferred embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1a is a schematic sectional view of the quadrupleclad
transmission fibre 10 comprising a core 11 and four cladding layers
12, 13, 14 and 15. FIG. 1b represents the variation of the
refractive index of such a fibre as a function of the radius. A
suitable choice of the radii and refractive indices of the core and
the cladding layers ensures that the fibre 10 has a low chromatic
dispersion over a comparatively wide range of wavelengths. The
comparatively low coupling efficiency of such a fibre is improved
by the invention.
The optical transmission system in accordance with the invention
shown in FIG. 2 comprises a light source 30, for example a diode
laser or a light-emitting diode. The radiation emitted by the
source is coupled into the core 11 of the quadruple-clad monomode
transmission fibre 10 via a coupling path I, said fibre 10 forming
the beginning of a long-distance transmission system. The coupling
path I comprises a single-clad monomode optical transmission fibre
20 of the single-step-index type, i.e. the cladding of this fibre
has only one refractive index. Since the core 21 of this fibre is
tapered at the end facing the light source, this fibre exhibits a
high coupling efficiency. A further increase in coupling efficiency
can be obtained by arranging a lens 22 on the tapered end portion,
for example in a manner as described in said article in
"Electronics Letters" Vol. 19, pp. 205-207. The lens 22 may be
provided with an anti-reflection coating 23. The refractive index
of the lens material may be substantially higher than the
refractive index of the core material of the fibre 20 in order to
obtain a large numerical aperture. Alternatively, the lens may be
made of a material whose refractive index is substantially equal to
the refractive index of the core material but whose melting point
is substantially lower than that of the core material. When the
lens 22 is attached care must be taken that the core material of
the fibre 20 is not softened.
The radiation transmitted by the transmission fibre 20 is
transferred to the multiple-clad fibre 10 at the location 45. Care
is taken that the spot diameters at the ends of the fibres 20 and
10 are equal to each other insofar as possible. In one embodiment
the cladding of the fibre 20 is therefore made of a material having
a refractive index which is approximately 1% smaller than the
refractive index of the core material of this fibre.
Suitably, the refractive indices of the core materials of the
fibres 20 and 10 are equal to each other as far as possible, so as
to minimize reflections from the transition between the fibre 20
and the fibre 10.
The mechanical coupling of the fibre 20 to the fibre 10 can be
established in known manner, for example as described in the
article "Progress in Monomode Optical Fibre Interconnection
Devices", in "Journ. of Lightwave Techn." Vol. LT 2, June 1984, pp.
217-227.
* * * * *